translate using map, C++ (vc++)

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//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64

#include <iostream>
#include <string>
#include <unordered_map>
#include <algorithm> // for transform()
#include <cassert> // <assert.h>
#include <cctype> // for std::tolower

std::string transToLower( std::string uIn );
std::string checkMap( std::string userInput );

int main() {

const std::string str = "HELLO world!" ;
    std::cout << str << '\n' ;

const std::string translated_str = checkMap(str) ;
    std::cout << translated_str << '\n' ;
}

// Converts user input string to lowercase.
std::string transToLower(std::string uIn) {

std::transform( uIn.begin(), uIn.end(), uIn.begin(),
                    []( char c ) { return std::tolower(c) ; } );
    return uIn;
}

std::string checkMap(std::string userInput) {

std::string result;

static const std::unordered_map<char, std::string> valueMap {
		{'a', "@"}, {'b', "8"}, {'c', "("}, {'d', "|)"},
		{'e', "3"}, {'f', "#"}, {'g', "6"}, {'h', "[-]"},
		{'i', "|"}, {'j', "_|"}, {'k', "|<"}, {'l', "1"},
		{'m', "[]\\/[]"}, {'n', "[]\\[]"}, {'o', "0"}, {'p', "|D"},
		{'q', "(,)"}, {'r', "|Z"}, {'s', "$"}, {'t', "']['"},
		{'u', "|_|"}, {'v', "\\/"}, {'w', "\\/\\/"}, {'x', "}{"},
		{'y', "`/"}, {'z', "2"}
	};

userInput = transToLower(userInput) ; // *** added

// for each char in userInput:
	for ( std::size_t i = 0; i < userInput.size(); ++i ) {

const char c = userInput[i];

// if c is found in valueMap (unordered_map) then do:
		if (valueMap.count(c) == 1)
			result += valueMap.at(c); // add valueMap.at(c) (the value in the map) to result string.
			// *** corrected: +=
		else
			result += result + c; // appends const char c to the result string
			// *** corrected: +=

}

// Need to update the return once done
	return result;
}

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Compilation time: 2,05 sec, absolute running time: 0,32 sec, absolute service time: 2,39 sec